docs: correct view listing cost and say what an expression may use

Two pages said list_materialized_views reads every table's schema and
that SHOW MATERIALIZED VIEWS is cheaper. That is the local
implementation; on a db:// connection the SDK asks the deployment for
its views, as SHOW MATERIALIZED VIEWS does.

The computed-column page now says that a literal must fit the column it
meets, and that a deployment's refresh can fail on an expression reading
a struct field, which the embedded library computes, so an expression
for a deployment should use top-level columns.

docs/web/sql/ddl/materialized-view.mdx

@@ -36,8 +36,8 @@ SHOW MATERIALIZED VIEWS
 ```
 
 <Note>
-Cheaper than the SDK equivalent. `list_materialized_views` finds views by
-reading every table's schema, costing one open per table; this asks the server.
+This asks the server for its views, as the SDK's `list_materialized_views` does
+on a `db://` connection.
 </Note>
 
 ## Refreshing one {#refresh}